2 Answers
2

Because Word of Life is an immediate reaction rather than an immediate interrupt, your ally is already at 0 or fewer hit points (and dying, unconscious, prone, etc.) if you choose to use it. After you use it (assuming they spend the healing surge), they're no longer dying. It doesn't interrupt dying, it just ends the dying condition by removing the state that caused it.

Rules Compendium, pages 195-196

An immediate interrupt jumps in when its trigger occurs, taking place before the trigger finishes.

An immediate reaction lets a creature act in response to a trigger. The triggering action or event occurs and is completely resolved before the action takes place. An immediate reaction waits for its trigger to finish, not necessarily for the action that contains the trigger to finish.

Word of Life does not save an ally from a hit that would kill them outright. It does give the attacker a -5 to all defenses, allowing some retribution, though.

Any event triggered with when a trigger occurs happens, happens during the same time frame as the triggering event. This helps distinguish When from After. It also makes sure that there are less problems with weird edge cases from ordering reactions and rulebook effects.

On the ground, it means that both the triggering and triggered event have to start after all Interrupts happen and before all Reactions start.

So no, Word of Life leaves you prone on the floor with hit points. Probably with a headache and some confusion from the brief moment of unconsciousness, but there's no rules for that.